Effect of two video cards in Mac Pro

[Grimace]

Hey gang, I have a Mac Pro and two video cards, and am upping the setup to a new Nehalem Mac Pro with the ATI 4780 card + NVidia GT120 card. For illustration purposes, I will use those two cards to frame my question, as we seem to be pretty familiar with their specs by now. My question is somewhat conceptual, and is difficult to explain, but I am counting on the MR collective intelligence to sort it out! I'll use a few scenarios to illustrate my point:

Scenario 1: Heavy use of the 4870 card, using both outputs (two screens). Naturally, performance is better than using the GT120 card.

Scenario 2: Same heavy use, but this time one port from the 4870 is used, and one port from the GT120.

Scenario 3: Same heavy use, but both ports on the 4870 are used, and one port on the GT120 (three screens total)

Scenario 3: Same heavy use, but both ports on the GT120 are used, and one port on the 4870 (three screens total)

How do software applications deal with two video cards? Would running Aperture, FCP, gaming etc., degrade just one screen? Wouldn't the whole application have to slow down to accommodate the lesser card?

Is there a "primary" card and a "secondary" card designation? I'm thinking of multiple monitor setups (three, for example) where a lesser card is necessary to drive the extra screen, but the application is using all three. 'Hopefully that makes some sense...

 

[Tallest Skil]

When I get mine, I'll find out for you.

 

[Grimace]

Thanks, I was hoping someone would have a general idea about how software programs behave with multiple cards and screens, but I'll take your impressions in a few weeks.

 

[Tallest Skil]

Thanks, I was hoping someone would have a general idea about how software programs behave with multiple cards and screens, but I'll take your impressions in a few weeks.

I'm assuming that there won't be too much lag, at least in OS X. Perhaps a little dragging a 1080p video from screen to screen.

I'll have two identical 24" monitors, one plugged into each, so I should get a fair estimation of the abilities of both.

 

[Grimace]

I'm assuming that there won't be too much lag, at least in OS X. Perhaps a little dragging a 1080p video from screen to screen.

I'll have two identical 24" monitors, one plugged into each, so I should get a fair estimation of the abilities of both.

I guess that is another way to estimate performance -- if you need solid performance on two monitors, should you go with two cards (high, low) or just one card (high). If the heavy lifting is divided between the cards, maybe each one can handle its assignment.

 

[barkmonster]

I've always used 1 video card per display and my current mac is set up like this:

On-board video (the faster, AGP Radeon card):

VGA 1280 x 1024 32bit Colour

PCI video card:

VGA 1024 x 768 16 bit Colour
50Hz TV 768 x 576 32 bit Colour

I might be imagining things but I'm sure if I push the TV up to 1024 x 768 too and run the VGA monitor in 32 bit colour, my system seems a tiny bit less snappy but it's nothing compared with how poor the video was on my beige G3 with just 1 large display!

When I started using a display at 1280 x 1024 even in 16 bit colour, the old G3 had such a performance hit before I got a decent video card to drive it that Pro Tools LE was unusable, it would just run out of juice straight away.

I bought a faster video card and the problem went away, it also made a huge difference to Photoshop, it was almost smoother than my current mac is at zooming around large images at high magnification and I used the on-board video to drive the ancient Multiple Scan Display at 1024 x 768 as a second display.

I'm going to hate going back to 1 display when I gets Mac Mini as a tie over system!

 

[trainguy77]

We learned a good bit about this back when the first mac pro came out. As if you had one ATI and one Nvidia card it could cause crashing etc. I don't know if this has been fixed.

But what people discovered is if you move something that directly uses the GPU from one screen to another it would cause lag or lockups. Which tells us that whatever monitor the application is on decides which GPU it uses. However, it used to be it couldn't switch between them on the fly. So applications like itunes when in visualizer would cause lag etc. Anything that does GPU processing would cause this problem.

Now if its fixed I don't know, but it shows us a little bit about this. So it depends if you run more then one GPU based heavy lifting application.

 

[gotzero]

I use multiple 2600 cards and never have any problems moving things from screen to screen.

 

[Tallest Skil]

I use multiple 2600 cards and never have any problems moving things from screen to screen.

I think he's just talking about having cards of two drastically different categories, such as the 4870 and the GT 120.

 

[Grimace]

But what people discovered is if you move something that directly uses the GPU from one screen to another it would cause lag or lockups. Which tells us that whatever monitor the application is on decides which GPU it uses. However, it used to be it couldn't switch between them on the fly. So applications like itunes when in visualizer would cause lag etc. Anything that does GPU processing would cause this problem.

Now if its fixed I don't know, but it shows us a little bit about this. So it depends if you run more then one GPU based heavy lifting application.

Exactly what I was looking for - thanks! I guess my question was whether the heavy lifting was being done by only the card powering the display that the application was in. That explains a lot.

Assuming this correct, I am guessing that folks who use more than three monitors, or who do heavy lifting on two - should choose their applications (and the monitors they are associated with) accordingly.